Current Class-A CMOS operational amplifiers include output stages that source or sink dc current to a load based upon the difference between the output and input signals. For example, if the output signal is more negative than the input signal then, the output stage of the amplifier sources or supplies more current to the load. Conversely, if the output signal is more positive than the input signal, then the output stage of the amplifier sinks current or pulls current away from the load. Typically, Class-A CMOS operational amplifiers are able to either source or sink current very well but cannot provide both good sourcing and sinking of current. This is especially evident where there is a large disparity between the input and output signals. Consequently, the current Class-A CMOS amplifiers exhibit a relatively high slew rate in one direction (sourcing/sinking) and a relatively low slew rate in the opposite direction (sinking/sourcing). Slew rate is defined as the time rate of change of the closed-loop amplifier output voltage under large-signal conditions, i.e. where there is a large disparity between the input and output signals.
Current CMOS operational amplifier design also requires a dominant S-plane pole to stabilize the amplifier and thereby prevent oscillation. Typically, the dominant S-plane pole is realized by an internal capacitor of 10 to 20 picofarads. On-chip capacitors are limited in size and their presence adds complexity, and hence expense, to the CMOS operational amplifier. Further, the size limitation of on-chip capacitors limit the CMOS operational amplifier's ability to filter noise. There is also an inverse relationship between the size of the capacitor and the CMOS operational amplifier's ability to respond under large signal conditions. Hence the capacitor affects the slew rate. For example, the higher the capacitance used to determine the dominant S-plane pole, the lower the ability of the CMOS operational amplifier to respond to a large disparity between the input and output signals.
A possible solution to the inability of current CMOS operational amplifiers to provide a high slew rate in both directions is to employ a Class-B amplifier rather than a Class-A amplifier. However, Class-B amplifiers are extremely hard to bias and maintain in a linear operation. Another possible solution is to employ a bi-polar output circuit connected to the CMOS operational amplifier. However, a bi-polar circuit would consume more power, take up more space, and dissipate more heat, thereby requiring more cooling and reducing the reliability of the circuit. Further, bipolar circuits are typically not available with the high input impedence CMOS devices.
Consequently, there exists a need for a Class-A CMOS operational amplifier having a high slew rate in both directions as characterized by its ability to both source and sink current well when there is a large difference between the input and output signals associated with the amplifier and/or a high capacitive load.